Midsole for footwear

ABSTRACT

A flexible innersole, or component thereof, for footwear of a foamed thermoplastic multi-block copolymer elastomer having a substantially uniform closed cell structure and having a specific gravity of less than about 0.35, an energy return ratio greater than about 0.55 when compressed and released said foam being prepared from a thermoplastic multi-block copolymer elastomer having a Shore D hardness of 25-45.

BACKGROUND OF THE INVENTION

This invention relates to a low density, closed cell foamedthermoplastic multi-block copolymer elastomer for use as midsoles forathletic shoes.

Midsoles for athletic shoes have been characterized as the mostimportant part of footwear. This portion of the shoe is the interfacebetween the foot of the wearer and the ground. The rather significantforce at which the foot of a runner strikes the ground, especially atthe toe and heel portions, is transmitted through the body of runner.The force developed is absorbed in significant part by the midsole ofthe shoe. The midsole returns the impact energy to the body and createsin the runner a beneficial sensation of springiness. Energy returnratios generated by currently used midsoles in athletic shoes aredeficient. Also, usually the midsoles of athletic running shoes losetheir weight bearing capacity after about two hundred fifty-miles ofuse. This is due to the large number of compression and return cyclesthat the midsole is subjected to which results in a considerablereduction in cushioning effect. A need in the industry exists for a lowdensity, foamed polymer that exhibits excellent shock attenuationcharacteristics and that has the property of storing elastic energy inthe midsole and returning a substantial portion of that energy, i.e.,energy return, which gives the runner a sensation of springiness. Also,there is need for midsoles in athletic shoes that do not break downafter extended use due to loss of weight bearing capacity, i.e.,fatigue, thus shortening the useful life of the shoe.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of an athletic shoe;

FIG. 2 is a sectional view of the shoe taken on a lengthwise midline2--2 of FIG. 1 showing the foamed thermoplastic elastomeric midsole ofthe shoe;

FIG. 3 is a partial sectional view of an elastomeric component of themidsole formed in the ball section of the midsole; and

FIG. 4 is a partial sectional view of an elastomeric component of themidsole formed in the heel section of the midsole.

SUMMARY OF THE INVENTION

It has now been discovered that certain thermoplastic multi-blockcopolymer elastomers that can be foamed .,to a low density areespecially suitable for use as midsoles, or components thereof, forathletic shoes. More specifically, the present invention is directed toa flexible midsole, or component thereof, for footwear, said midsolecomprising a foamed thermoplastic multi-block copolymer elastomer havinga substantially uniform closed cell structure having a specific gravityof less than about 0.35, an energy return ratio greater than about 0.55,preferably greater than about 0.60, when compressed and released, asdetermined by the recommended method of ASTM Committee OF8.54 onathletic footwear, said foam being prepared from a thermoplasticmulti-block copolymer elastomer having a Shore D hardness of from about25-45 and selected from the group consisting of (1) copolyetheresters,(2) copolyesteresters, (3) copolyetherimide esters, and (4)copolyetheramides, said thermoplastic foam being prepared by mixing saidmulti-block copolymer elastomer at a temperature above its melting pointto form a molten mass with a gaseous or low-boiling liquid foaming agentat a pressure sufficient to dissolve and/or disperse said foaming agentin said molten elastomer and passing the resulting mixture through anorifice into a lower pressure zone whereupon foaming and expansion occurand the elastomer solidifies.

The thermoplastic multi-block copolymer elastomers used to make themidsoles preferably have a melt index at foaming temperatures notgreater than 10 g/10 minutes by the procedure described in ASTM D1238(2.16 kg wt.). Most preferably, the thermoplastic multi-block elastomerhas a melt index no greater that about 6 g/10 minutes.

The thermoplastic multi-block copolymer elastomers used to make themidsoles have a Shore D hardness of from 25-45. Preferably, the Shore Dhardness of the thermoplastic elastomer is from 30-43. Thermoplasticelastomers having a Shore D hardness greater than 45 result in a foamedelastomer that does not have sufficient flexibility for use as anmidsole in athletic footwear, and those having a Shore D hardness ofless than about 25 yield foams that do not have sufficient weightbearing capacities.

Preferably, the thermoplastic multi-block copolymer elastomers used tomake the midsoles are copolyetheresters or copolyetheramide esters,especially copolyetheresters.

The flexible midsoles of the present invention have a surprisingly highenergy return ratio which is an important characteristic for athleticshoes.

DETAILED DESCRIPTION OF THE INVENTION

Turning to the drawings, FIG. 1 shows a perspective view of an athleticrunning shoe 10 comprising the upper 11 of the shoe that fastens theshoe to the wearer and provides support. The thermoplastic foamedmidsole 12, shown in cross-section in FIG. 2, is bonded by cement,usually a polyurethane or neoprene-based cement, to outersole 13.Outersole 13 provides traction for the wearer and is usually made of athermoset rubber, such as a styrene-butadiene rubber, polyurethane ornatural rubber.

It is conventional practice, in the athletic shoe industry to constructan midsole out of one polymer or, alternatively, the midsole can beconstructed out of a combination of different polymers. The benefits ofthe present invention are obtained if midsole 12 is constructed from thethermoplastic foamed copolymer or, as shown in FIGS. 3 and 4, thethermoplastic foamed copolymer can be a component of midsole 12. Forexample, midsole component 12' is shown in FIG. 3 inserted in the ballof the wearer's foot section where forceful contact of the shoe is madewith the ground. The other portions of the midsole can be fabricatedfrom a conventional polymer foam, e.g., an ethylene vinyl acetatecopolymer. In like manner, FIG. 4 illustrates foamed thermoplasticelastomer midsole component 12" in the heel portion of the shoe whereforceful contact of the shoe is made with the ground. The shoes arefurther assembled by a conventional procedure called lasting. Finally,outersole 13 and midsole 12 are trimmed to size and the soles attachedto upper 11 by cement. The benefit of the invention in term of energyreturn is obtained when a component of midsole 12 made from athermoplastic multi-block copolymer elastomer is fabricated in the balland/or heel area of an midsole made from another polymer where forcefulcontact of the sole is made with the ground by the wearer.

The thermoplastic multi-block copolymer elastomers that are used in thisinvention to form a flexible thermoplastic foam are(a)copolyetheresters, (b) copolyesteresters, (c) copolyetherimideesters, and (d) copolyetheramides. The four types of elastomers aresimilar to one another in that they all consist of repeating hardsegments which are relatively high melting polyester or polyamidesegments and repeating soft segments which are relatively low meltingpolyether or polyester segments. The four types of polymers describedbelow are well known in the industry.

The copolyetheresters (a) consist essentially of a multiplicity ofrecurring long chain ester units and short chain ester units joinedhead-to-tail through ester linkages, said long chain ester units beingrepresented by the formula ##STR1## and said short chain ester unitsbeing represented by the formula where G is a divalent radical remainingafter the removal of terminal hydroxyl groups from a poly(alkyleneoxide) glycol having a number average molecular weight of about 400-6000and a carbon to oxygen atomic ratio of about 2.0-4.3; R is a divalentradical remaining after removal of carboxyl groups from an aromaticdicarboxylic acid having a molecular weight less than about 300, and Dis a divalent radical remaining after removal of hydroxyl groups from adiol having a molecular weight less than about 250; provided said shortchain ester units amount to about 25-70 percent by weight of saidcopolyetherester.

The term "long-chain ester units" as applied to units in a polymer chainof the copolyetherester refers to the reaction product of a long-chainglycol with a dicarboxylic acid. Such "long-chain ester units", whichare a repeating unit in the copolyetheresters, correspond to formula (I)above. The long-chain glycols are polymeric glycols having terminal (oras nearly terminal as possible) hydroxy groups and a molecular weightfrom about 400-6000. The long-chain glycols used to prepare thecopolyetheresters are poly(alkylene oxide) glycols having acarbon-to-oxygen atomic ratio of about 2.0-4.3. Representativelong-chain glycols are poly(ethylene oxide) glycol, poly(1,2- and1,3-propylene oxide) glycol, poly(tetramethylene oxide) glycol, randomor block copolymers or ethylene oxide and 1,2-propylene oxide, andrandom or block copolymers of tetrahydrofuran with minor amounts of asecond monomer such as ethylene oxide.

The term "short-chain ester units" as applied to units in a polymerchain of the copolyetherester refers to low molecular weight chain unitshaving molecular weights less than about 550. They are made by reactinga low molecular weight diol (below about 250) with an aromaticdicarboxylic acid having a molecular weight below about 300, to formester units represented by formula (II) above.

The term "low molecular weight diols" as used herein should be construedto include equivalent ester-forming derivatives, provided, however, thatthe molecular weight requirement pertains to the diol only and not toits derivatives.

Preferred are diols with 2-15 carbon atoms such as ethylene, propylene,tetramethylene, pentamethylene, 2,2-dimethyltrimethylene, hexamethylene,and decamethylene glycols, dihydroxycyclohexane, cyclohexane dimethanol,and the unsaturated 1,4-butenediol.

The term "dicarboxylic acids" as used herein, includes equivalents ofdicarboxylic acids having two functional groups which performsubstantially like dicarboxylic acids in reaction with glycols and diolsin forming copolyetherester polymers. These equivalents include estersand ester-forming derivatives, such as acid anhydrides. The molecularweight requirement pertains to the acid and not to its equivalent esteror ester-forming derivative.

Among the aromatic dicarboxylic acids for preparing the copolyetheresterpolymers, those with 8-16 carbon atoms are preferred, particularly thephenylene dicarboxylic acids, i.e., phthalic, terephthalic andisophthalic acids and their dimethyl esters.

The short-chain ester units will constitute about 25-70 weight percentof the copolyetherester. The remainder of the copolyetherester will belong-chain ester units comprising about 30-75 weight percent of thecopolyetherester.

Preferred copolyetheresters are those prepared from dimethylterephthalate, 1,4-butanediol, and poly(tetramethylene oxide) glycolhaving a molecular weight of about 600-2000. Optionally, up to about 30mole percent of the dimethyl terephthalate in these polymers can bereplaced by dimethyl phthalate or dimethyl isophthalate. Polymers inwhich a portion of the butanediol is replaced by butenediol are alsopreferred.

The dicarboxylic acids or their derivatives and the polymeric glycol areincorporated into the copolyetherester in the same molar proportions asare present in the reaction mixture. The amount of low molecular weightdiol actually incorporated corresponds to the difference between themoles of diacid and polymeric glycol present in the reaction mixture.When mixtures of low molecular weight diols are employed, the amounts ofeach diol incorporated depends on their molar concentration, boilingpoints and relative reactivities. The total amount of diol incorporatedis still the difference between moles of diacid and polymeric glycol.

The copolyetheresters described herein are made by a conventional esterinterchange reaction which, preferably, takes place in the presence of aphenolic antioxidant that is stable and substantially nonvolatile duringthe polymerization.

A preferred procedure involves heating the dimethyl ester ofterephthalic acid with a long-chain glycol and 1,4-butanediol in a molarexcess and a phenolic antioxidant and hindered amine photostabilizer ineffective concentrations in the presence of a catalyst at about150°-260° C. and a pressure of 0.05 to 0.5 Mpa, preferably ambientpressure, while distilling off methanol formed by the ester interchange.Depending on temperature, catalyst, glycol excess and equipment, thisreaction can be completed within a few minutes, e.g., about two minutes,to a few hours, e.g., about two hours. This procedure results in thepreparation of a low molecular weight prepolymer which can be carried toa high molecular weight copolyetherester by distillation of the excessof short-chain diol. The second process stage is known as"polycondensation".

Additional ester interchange occurs during this polycondensation whichserves to increase the molecular weight and to randomize the arrangementof the copolyetherester units. Best results are usually obtained if thisfinal distillation or polycondensation is run at less than about 670 Pa,preferably less than about 250 Pa, and about 200°-280° C., preferablyabout 220°-260° C., for less than about two hours, e.g., about 0.5 to1.5 hours. A phenolic antioxidant can be introduced at any stage ofcopolyetherester formation or after the polymer is prepared. Asindicated above, preferably, a phenolic antioxidant is added with themonomers. It is customary to employ a catalyst while carrying out esterinterchange reactions. While a wide variety of catalysts can beemployed, organic titanates such as tetrabutyl titanate used alone or incombination with magnesium or calcium acetates are preferred. Thecatalyst should be present in the amount of about 0.005 to 2.0 percentby weight based on total reactants.

Both batch and continuous methods can be used for any stage ofcopolyetherester polymer preparation. Polycondensation of prepolymeralready containing the phenolic antioxidant and hindered aminephotostabilizer can also be accomplished in the solid phase by heatingdivided solid prepolymer in a vacuum or in a stream of inert gas toremove liberated low molecular weight diol. This method has theadvantage of reducing thermal degradation because it must be used attemperatures below the softening point of the prepolymer.

A more detailed description of suitable copolyetheresters and proceduresfor their preparation are further described in U.S. Pat. Nos. 3,023,192,3,651,014, 3,763,109, 3,766,146, and 4,355,155, the disclosures of whichare incorporated herein by reference.

The copolyesteresters (b) consist essentially of high melting segmentscomprised of repeating short-chain ester units of the formula ##STR2##are as described for copolyetheresters as disclosed hereinbefore. Thesoft segments in the copolyesterester elastomers are derived from lowmelting polyester glycols such as poly(butylene adipate) orpoly(caprolactone).

Several procedures have been used to prepare multi-blockcopolyesterester elastomers wherein the low melting point blocks arepolyesters as well as the high melting point blocks. One procedureinvolves carrying out a limited ester interchange reaction in thepresence of an exchange catalyst between two high molecular weightpolymers such as poly(butylene terephthalate) and poly(butyleneadipate). Ester exchange at first causes the introduction of blocks ofone polyester in the other polyester chain and vice versa. When thedesired multi-block polymer structure is formed the catalyst isdeactivated to prevent further interchange which ultimately would leadto a random copolyester without any blockiness. This procedure isdescribed in more detail in U.S. Pat. No. 4,031,165 to Saidi et al.Other useful procedures involve coupling of preformed blocks of high andlow melting point polyester glycols. Coupling can be accomplished byreaction of a mixture of the blocks with a diisocyanate as described inEuropean Pat. No. 00013461 to Huntjens et al. Coupling can also beaccomplished by heating the mixed blocks in the presence ofterephthaloyl or isophthaloyl bis-caprolactam addition compounds. Thecaprolactam addition compounds react readily with the terminal hydroxylgroups of the polyester blocks, splitting out caprolactam and joiningthe blocks through ester linkages. This coupling method is furtherdescribed in Japanese Patent Publication No. 73/4115. Another procedureof use when the low melting blocks are to be provided bypolycaprolactone involves reacting a preformed high melting point blockterminated with hydroxyl groups with epsilon-caprolactone in thepresence of a catalyst such as dibutyl tin dilaurate. The caprolactonepolymerizes on the hydroxyl groups of the high melting point ester blockwhich groups serve as initiators. The resulting product is a relativelylow molecular weight triblock polymer having the high melting pointblock in the middle with low melting point polycaprolactone blocks oneach end. The triblock polymer is hydroxyl terminated and may be joinedto give a finished product by reaction with a diepoxide such asdiethylene glycol diglycidyl ether, see Japanese Patent Publication No.83/162654.

The copolyetherimide ester elastomers (c) differ from thecopolyetheresters (a) only in that repeating hard segments and softsegments are joined through imidoester linkages rather than simple esterlinkages. The hard segments in these elastomers consist essentially ofmultiple short chain ester units represented by the formula ##STR3##hereinbefore. The soft segments in these polymers are derived frompoly(oxyalkylene diimide) diacids which can be characterized by thefollowing formula: ##STR4## wherein each R" is independently a trivalentorganic radical, preferably a C₂ to C₂₀ aliphatic, aromatic orcycloaliphatic trivalent organic radical; each R' is independentlyhydrogen or a monovalent organic radical preferably selected from thegroup consisting of C₁ to C₆ aliphatic and cycloaliphatic radicals andC₆ to C₁₂ aromatic radicals, e.g, benzyl, most preferably hydrogen; andG' is the radical remaining after the removal of the terminal (or asnearly terminal as possible) amino groups of a long chain ether diaminehaving an average molecular weight of from about 600 to about 12000,preferably from about 900 to about 4000, and a carbon-to-oxygen ratio offrom about 1.8 to about 4.3

Representative long chain ether glycols from which the polyoxyalkylenediamine is prepared include poly (ethylene ether)glycol; poly(propyleneether)glycol; poly(tetramethylene ether) glycol; random or blockcopolymers of ethylene oxide and propylene oxide, including propyleneoxide terminated poly(ethylene ether) glycol; and random or blockcopolymers of tetrahydrofuran with minor amounts of a second monomersuch as methyl tetrahydrofuran (used in proportion such that thecarbon-to-oxygen mole ratio in the glycol does not exceed about 4.3).Especially preferred poly(alkylene ether) glycols are poly(propyleneether) glycol and poly(ethylene ether) glycols end capped withpoly(propylene ether) glycol and/or propylene oxide.

In general, the polyoxyalkylene diamines will have an average molecularweight of from about 600 to 12000, preferably from about 900 to about4000.

The tricarboxylic component is a carboxylic acid anhydride containing anadditional carboxylic group or the corresponding acid thereof containingtwo imide-forming vicinal carboxyl groups in lieu of the anhydridegroup. Mixtures thereof are also suitable. The additional carboxylicgroup must be esterifiable and preferably is substantiallynonimidizable.

Further, while trimellitic anhydride is preferred as the tricarboxyliccomponent, any of a number of suitable tricarboxylic acid constituentswill occur to those skilled in the art including 2,6,7 naphthalenetricarboxylic anhydride; 3,3',4- diphenyl tricarboxylic anhydride;3,3'4-benzophenone tricarboxylic anhydride; 1,3,4-cyclopentane otricarboxylic anhydride; 2,2',3-diphenyl tricarboxylic anhydride;diphenyl sulfone-3,3',4-tricarboxylic anhydride, ethylene tricarboxylicanhydride; 1,2,5-naphthalene tricarboxylic anhydride; 1,2,4-butanetricarboxylic anhydride; diphenyl isopropylidene 3,3',4-tricarboxylicanhydride; 3,4-dicarboxyphenyl 3'-carboxylphenyl ether anhydride;1,3,4-cyclohexane tricarboxylic anhydride; etc. These tricarboxylic acidmaterials can be characterized by the following formula: ##STR5## whereR" is a trivalent organic radical, preferably a C₂ to C₂₀ aliphatic,aromatic, or cycloaliphatic trivalent organic radical and R' ispreferably hydrogen or a monovalent organic radical preferably selectedfrom the group consisting of C₁ to C₆ aliphatic and/or cycloaliphaticradicals and C₆ to C₁₂ aromatic radicals, e.g., benzyl; most preferablyhydrogen.

Briefly, the polyoxyalkylene diimide diacids (III) may be prepared byknown imidization reactions including melt synthesis or by synthesizingin a solvent system. Such reactions will generally occur at temperaturesof from 100-300° C., preferably at from about 150° C. to about 250° C.while drawing off water or in a solvent system at the reflux temperatureof the solvent or azeotropic (solvent) mixture.

Although the weight ratio of the above ingredients is not critical, itis preferred that the diol be present in at least a molar equivalentamount, preferably a molar excess, most preferably at least 150 molepercent based on the moles of dicarboxylic acid and polyoxyalkylenediimide diacid combined. Such molar excess of diol will allow foroptimal yields, based on the amount of acids, while compensating for theloss of diol during esterification/condensation.

Further, while the weight ratio of dicarboxylic acid to polyoxyalkylenediimide diacid is not critical to form the polyetherimide esters,preferred compositions are those in which the weight ratio of thepolyoxyalkylene diimide diacid to dicarboxylic acid is from about 0.25to about 2, preferably from about 0.4 to about 1.4. The actual weightratio employed will be dependent upon the specific polyoxyalkylenediimide diacid used and more importantly, the desired physical andchemical properties of the resultant polyetherimide ester. In general,the lower the ratio of polyoxyalkylene diimide diester to dicarboxylicacid the better the strength, crystallization and heat distortionproperties of the polymer. Alternatively, the higher the ratio, thebetter the flexibility, tensile set and low temperature impactcharacteristics.

Generally, the thermoplastic elastomers comprise the reaction product ofdimethylterephthalate, optimally with up to 40 mole percent of anotherdicarboxylic acid; 1,4-butanediol, optionally with up to 40 mole percentof another saturated or unsaturated aliphatic and/or cycloaliphaticdiol; and a polyoxyalkylene diimide diacid prepared from apolyoxyalkylene diamine of molecular weight of from about 600 to about12,000, preferably from about 900 to about 4000, and trimelliticanhydride. The diol can be 100 mole percent 1,4-butanediol and thedicarboxylic acid 100 mole percent dimethylterephthalate.

The polyetherimide esters described herein may be prepared byconventional esterification/condensation reactions for the production ofpolyesters. Exemplary of the processes that may be practiced are as setforth in, for example, U.S. Pat. Nos. 3,023,192; 3,763,109; 3,651,014;3,663,653 and 3,801,547, incorporated herein by reference.

The preparation of the copolyetherimide ester is more fully described inU.S. Pat. No. 4,556,705, incorporated herein by reference.

The copolyetheramide elastomers (d) differ from the three types ofelastomers previously described in that their recurring hard segmentsare based on repeating amide units rather than short chain ester units.The repeating amide units may be represented by the formula: ##STR6## orby the formula

    --HN--R'''--NHCOR''''CO--                                  (VI)

wherein L is a divalent hydrocarbon radical containing 4-14 carbonatoms, R''' is a divalent hydrocarbon radical of 6-9 carbon atoms andR'''' is a divalent hydrocarbon radical of 6-12 carbon atoms.

The hard segments for these polymers are normally prepared in a separatestep in which a suitable lactam or omega-amino acid or a nylon salt isheated in the presence of a minor amount of a dicarboxylic acid whichcontrols the molecular weight of the polyamide oligomer formed. In asecond step, the acid-terminated amide hard segments are mixed with anequivalent amount of a poly(alkylene oxide) glycol and heated in thepresence of a titanate catalyst to form the elastomer. The glycolprovides the soft segments in the polymer. The soft segments can berepresented by the formula

    --OGO--                                                    (VII)

wherein G is a divalent radical remaining after the removal of terminalhydroxy groups from a poly(alkylene oxide) glycol having an averagemolecular weight of about 400-3500. The hard and soft segments arejoined through ester linkages. The resulting polymer has an intrinsicviscosity of about 0.8-2.05. In block copolymers with elastomericproperties the average molecular weight of the polyamide sequencespreferably is in the range of from about 500 to 3000, most preferablyfrom abut 500 to about 2000. In block copolymers with elastomericproperties, the average molecular weight of the polyoxyalkylene glycolmay vary from about 400 to about 6000, preferably from about 500 toabout 5000, most preferably from about 400 to about 3000, in particularfrom about 1000 to about 3000.

The proportion by weight of the polyoxyalkylene glycol with respect tothe total weight of the polyetheramide block copolymer can vary fromabout 5% to about 90%, suitably from about 5% to about 85%.

These polymers and their preparation are described in greater detail inU.S. Pat. No. 4,331,786, incorporated herein by reference.

The thermoplastic elastomer compositions of this invention are foamedinto large slabs having a cross-sectional area of the order of six toten square inches or larger and about one to two inches thick. Thefoamed thermoplastic elastomers have a very low specific gravity of lessthan about 0.35, usually 0.15-0.30 and have energy return ratio valuesof greater than about 0.55, preferably 0.60. The slabs of foam made fromthe thermoplastic elastomer compositions have a closed cell structurethat is substantially uniform.

The foamed slabs can be made by adding the thermoplastic multi-blockcopolymer elastomer, usually in the form of pellets, to an extruderthrough a hopper. The elastomer is heated and masticated in the extruderto produce a molten mass of the elastomer that is mixed and advancedthrough the extruder. The temperature necessary to produce the moltenmass will vary with the particular elastomer to be foamed but generallyis within a range of 130°-230° C. The foaming agent is either injectedor incorporated in the thermoplastic elastomer and thoroughly mixed withthe molten elastomer as it advances through the extruder. The mixture ofmolten elastomer and foaming agent is cooled as it advances through theextruder. The mixture is cooled to a temperature at which the viscosityof the elastomer is adequate to retain the foaming agent when themixture is subjected to conditions of lower pressure and is allowed toexpand. After cooling, the mixture is extruded into a holding zonemaintained at a temperature and pressure that prevents foaming of themixture. The holding zone has an outlet die having an orifice openinginto a zone of lower pressure, such as atmosphere pressure, where themixture foams. The die orifice is externally closable by a gate. Themovement of the gate does not disturb the foamable mixture within theholding zone. The foamable mixture is extruded from the holding zone bya movable ram which forces the foamable mixture out of the holding zonethrough the die orifice at a rate greater than that at which substantialfoaming in the die orifice occurs and less than at which melt fractureoccurs. Generally, this ranges between about 1000-5000 lbs/hr.OptionallY, an extruder that is sufficiently large can be used toextrude the foamable melt through the die at a rate great enough toprevent foaming in the die orifice. Upon passing through the die orificeinto the zone of lower pressure, the foamable mixture is allowed toexpand unrestrained in at least one dimension to produce the desiredlarge size slab of foamed thermoplastic elastomer having a low specificgravity. Such method for foaming the thermoplastic multi-block copolymerelastomer to a low density is disclosed in U.S. Pat. No. 4,323,528, thedisclosure of which is incorporated herein by reference.

As also taught in U.S. Pat. No. 4,323,528, after substantially uniformand free expansion of the foaming mass has occurred, the hot cellularmass is still totally deformable and at that stage can be formed ifdisposed between two mold halves which are hot cellular mass. Becausethe cellular mass is still capable of limited further expansion while itis totally deformable, the foam mass fills the mold completely andaccurately reproduces the shape of the mold.

The foaming agents used to make the low density thermoplastic elastomercan be liquids, solids or inert gases. Suitable foaming agents arehydrocarbons and partially or fully halogenated hydrocarbons.Representative liquid foaming agents include halocarbons such asmethylene chloride, trichloromethane, dichlorofluoromethane,trifluoro-chloromethane, difluorotetrachloro-ethane,dichlorotetrafluoroethane, chlorotrifluoroethane, difluoro-ethane, andhydrocarbons such as butane, isobutane, pentane, hexane, and propane.Solid chemical foaming agents can also be used to foam the thermoplasticelastomers. Representative chemical foaming agents includediazodicarbon-amide and other azo, N-nitroso, carbonate and sulfonylhydrazides that decompose when heated. Also, inert gases can be used asfoaming agents such as nitrogen and carbon dioxide. The preferredfoaming agents are hydrocarbons, such as isobutane and halogenatedhydrocarbons.

The following examples further illustrate the invention.

Procedure for Determining Energy Return Ratio

The energy return ratio of the foams described in the examples wereobtained by a gravity driven drop test following the proposedrecommendations of ASTM Committee FO8.54 on Athletic Footwear.

This test was performed on an instrumented impact apparatus identifiedas DYNATUP, which is supplied by General Research Corporation. TheDynatup system used for these tests consists of a gravity driven dropweight machine and an IBM PC computer for analysis and presentation ofresults. This system measures the velocity at impact and the force-timerecord during impact. From this data, the program generates completerecords of force, deflection, energy, time, and velocity.

The test procedure involves dropping a 8.5 kg weight from a height of 5cm on to the sample. The shape of the surface contacting the sample(tup) was 46 mm diameter flat with chamfered edges. The foam samples arepositioned on a rigid steel anvil.

From the force-time and energy-time records stored in the computer, thecomputer program can calculate values for (1) the maximum energyimparted to the foam by the falling weight and (2) the energy returnedby the foam. The ratio of the energy returned by the foam to the maximumenergy imparted to the foam is defined as the "energy return ratio". Aperfect spring would have an energy return ratio of 1.00.

Polymers

The thermoplastic multi-block copolymer elastomers used in the exampleswere as follows:

Elastomer A is a copolyetherester elastomer containing 41.3% by weightbutylene terephthalate short chain ester units, 12% by weight butyleneisophthalate short chain ester units and 46.7% by weight long chainester units derived from ethylene oxide-capped poly(propylene oxide)glycol containing 30% by weight ethylene oxide units andterephthalic/isophthalic acids. The elastomer has a Shore D hardness of40 and a melt index at 190° C. of 3 g/10 minutes by ASTM D 1238 (2.16 kgweight).

Elastomer B has the same composition and hardness of Elastomer A, buthas a melt index at 190° C. of 7 g/10 minutes.

Elastomer C is a copolyetheramide elastomer containing 42% by weight ofpolyamide hard segments derived from lauryl lactam and 53% by weight ofsoft segments derived from poly(tetramethylene oxide) glycol. Theelastomer has a Shore D hardness of 40 and a melt index at 220° C. of1.2 g/10 minutes by ASTM D 1238 (2.16 kg weight).

Foam Preparation

The foams described in the following examples were prepared in a 3 inchdiameter, 48:1 extruder. The extruder was equipped with the apparatusnecessary to inject foaming agents and the forward portion of theextruder barrel was jacketed for cooling using circulating water. Theextruder was attached to a foam accumulator described in U.S. Pat. No.4,323,528. This accumulator is equipped with a piston for ejecting(extruding) the foamable melt through a closable die. The speed of thepiston can be varied to provide various extrusion rates. The use of anaccumulator is not necessary to produce foams of large cross-sectionswith a large extruder. However, its use was required with the relativelysmall foam extruder used in the examples, which, by itself, would beincapable of producing large cross-sections. The use of a relativelysmall extruder also conserved raw materials as the foamable melt wasextruded at rates of about 1000 to 5000 lbs/hour while the actual outputrate of the extruder was about 120 lbs/hour.

EXAMPLE 1

The foam accumulator was equipped with a bow-tie shaped die measuring1/8 inch thick in the center by 3" wide. Elastomer A was mixed at thehopper of the extruder with 0.1% of "Hydrocerol" CF for cell sizecontrol. ("Hydrocerol" is an encapsulated mixture of sodium bicarbonate,citric acid and citric acid salts which liberates carbon dioxide andwater under elevated temperatures in the extruder.) The foaming agent,an 80:20 mixture of dichlorotetrafluoroethane (CFC 114) anddichlorodifluoromethane (CFC 12) was injected at the rates shown inTable 1. The output of the extruder was about 120 lbs./hour. After thefoaming agent was injected, it was mixed into the polymer and then themixture was cooled to the proper foaming temperature, about 177° C. Thefoamable melt exiting the extruder was transferred under pressure to theaccumulator where it was stored and released intermittently at a rate of3000 lbs/hour. The foam planks produced were about one inch thick, had across-sectional area of 6 to 10 square inches and specific gravitiesshown in Table 1.

                  TABLE 1                                                         ______________________________________                                                     Foaming Agent Level                                                                          Specific                                          Number       (lbs./hour)    Gravity                                           ______________________________________                                        1            2              0.32                                              2            3              0.28                                              ______________________________________                                    

The foams produced had uniform fine cell structure and had an energyreturn ratio in excess of 0.55.

EXAMPLE 2

The same apparatus and elastomer used in Example 1 were used in thisexample. Talc was used as a nucleating agent instead of "Hydrocerol" CF.The foaming agents used are enumerated in Table 2 along with specificgravities of the foams prepared. The energy return ratio of the foams isalso given in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                                  Talc Level  Energy                                   Foaming Agent Foaming Agent Level                                                                      (% by weight                                                                         Specific                                                                           Return                              Number                                                                             Type          (lbs./hour)                                                                              of resin)                                                                            Gravity                                                                            Ratio                               __________________________________________________________________________    1    CFC-12/CFC-114 20:80                                                                        5          .15    0.24 0.597                               2    CFC-12/CFC-114 20:80                                                                        7          .15    0.19 0.778                               3    HCFC-142b     5          .15    0.21 --                                  4    HCFC-22/HCFC-142b 1.8:1                                                                     2.8        .30    0.25 0.812                               5    HCFC-22/HCFC-142b 1.8:1                                                                     2.8        .15    0.26 0.749                               6    HCFC-22       2.5        .15    0.25 0.648                               7    HCFC-22       2.5        .30    0.28 0.678                               __________________________________________________________________________     In the Table,                                                                 HCFC142b = chlorodifluoroethane                                               HCFC22 = chlorodifluoromethane                                                CFC114 = dichlorotetrafluoroethane                                            CFC12 = dichlorodifluoromethane                                          

Foam sample numbers 1 and 2 had a thin, smooth "skin" resulting from thecollapse of the outermost bubbles. The "skin" on foam numbers 3 through7 was thicker and somewhat irregular due to the escape of the foamingagent from the outermost portions of the foam before air could replaceit. As shown in Table 2, the foams exhibited good resiliency propertiesand were useful as midsoles in footwear. All samples were about one inchthick and had 6 to 10 square inches of cross-sectional area.

EXAMPLE 3

Using the same apparatus as in example 1, foams were produced usingElastomer A and Elastomer B with isobutane as the foaming agent.Hydrocarbons, such as isobutane, have low permeability throughElastomers A and B so foam produced using hydrocarbon foaming agentsshould exhibit minimal shrinkage and skin-formation as the foaming agentwill not leave the foam faster than air can replace it. The results areshown in Table 3.

                  TABLE 3                                                         ______________________________________                                                          Isobutane   Talc    Specific                                Number  Elastomer Level (pph) Level (%)                                                                             Gravity                                 ______________________________________                                        1       A         1.0         1.2     0.23                                    2       A         1.5         1.2     0.18                                    3       B         1.5         1.2     0.19                                    4       B         1.0         1.2     0.23                                    ______________________________________                                         pph = lbs./hour                                                          

Foams 1 though 4 had a uniform cell structure and a very thin smoothskin. All samples were suitable for use as midsoles in footwear and hadenergy return ratios in excess of 0.55. Better results were obtainedwith Elastomer B when it was foamed at a lower temperature thanElastomer A.

EXAMPLE 4

Using the same apparatus described in Example 1, foam samples wereprepared from Elastomer C using the ingredients shown in Table 4.Specific gravities of the resulting foams are also given in the table.

                  TABLE 4                                                         ______________________________________                                                Foaming  Foaming Agent                                                                              Talc    Specific                                Number  Agent    Level (pph)  Level (%)                                                                             Gravities                               ______________________________________                                        1       Isobutane                                                                              0.8          1.8     0.26                                    2       Isobutane                                                                              1.0          1.8     0.23                                    3       Isobutane                                                                              1.5          1.8     0.18                                    ______________________________________                                    

Foams 1 and 2 had uniform cell structures and thin smooth skins. Foam 3showed some evidence of voiding indicating a specific gravity of about0.18 is about the minimum for this elastomer. The foams have energyreturn ratios greater than 0.55 and were suitable of ruse in athleticfootwear midsoles.

We claim:
 1. A flexible midsole for footwear, said midsole comprising afoamed thermoplastic multi-block copolymer elastomer having asubstantially uniform closed cell structure and having a specificgravity of less than about 0.35, an energy return ratio greater thanabout 0.55 when compressed and released, as determined by therecommended method of ASTM Committee FO8.54 on athletic footwear, saidfoam being prepared from a thermoplastic multi-block copolymer elastomerhaving a Shore D hardness of 25-45 and selected from the groupconsisting of (1) copolyetheresters, (2) copolyesteresters, (3)copolyetherimide esters and (4) copolyetheramides, said thermoplasticfoam being prepared by mixing said multi-block copolymer elastomer at atemperature above its melting point to form a molten mass with a gaseousor low-boiling liquid forming agent at a pressure sufficient to dissolveand/or disperse said foaming agent in said molten elastomer andextruding the resulting mixture through an orifice into a lower pressurezone whereupon foaming and expansion occur and the elastomer solidifies.2. A flexible midsole of claim 1 wherein the multi-block copolymerelastomer is a copolyetherester.
 3. A flexible midsole of claim 2wherein the copolyetherester consists essentially of a multiplicity ofrecurring long chain ester units and short chain ester units joinedheat-to-tail through ester linkages, said long chain ester units beingpresented by the formula: ##STR7## and said short chain ester unitsbeing represented by the formula: ##STR8## where G is a divalent radicalremaining after the removal of terminal hydroxyl groups from apoly(alkylene oxide) glycol having an average molecular weight of about400-3500; R is a divalent radical remaining after the removal ofcarboxyl groups from a dicarboxylic acid having a molecular weight lessthan about 300; D is a divalent radical remaining after the removal ofhydroxyl groups from one or more saturated and/or unsaturated diolshaving a molecular weight less than about 250; with the proviso that theshort chain ester units constitute about 25-70% by weight of thecopolyetherester and and long chain ester units constitute about 30-76%by weight of the copolyetherester.
 4. A flexible midsole of claim 1wherein the multi-block copolymer elastomer is a copolyetheramide.
 5. Aflexible midsole of claim 4 wherein the copolyetheramide consistsessentially of recurring hard segments based on repeating amide unitsbeing represented by the formula: ##STR9## wherein L is a divalenthydrocarbon radical containing 4-14 carbon atoms, R''' is a divalenthydrocarbon radical of 6-9 carbon atoms and R'''' is a divalenthydrocarbon radical of 6-12 carbon atoms and recurring soft segmentsbeing represented by the formula

    --OGO--                                                    (VII)

where G is a divalent radical remaining after the removal of terminalhydroxy groups from a poly(alkylene oxide)glycol having an averagemolecular weight of about 400-3500, said hard and soft segments beingjoined through ester linkages.
 6. A flexible midsole of claim 1 whereinthe thermoplastic multi-block copolymer elastomers have a Shore Dhardness of from 30-43.
 7. A flexible midsole of claim 1 wherein thefoaming agent is a hydrocarbon or a partially or fully halogenatedhydrocarbon.
 8. A flexible midsole, of claim 1 wherein the energy returnratio is greater than about 0.60.